A polymer electrolyte fuel cell makes a supplied fuel gas such as hydrogen and an oxidant gas such as air react electrochemically on a catalyst such as platinum, thereby generating electric power and heat at the same time. In general, the electrode to which a fuel gas is supplied is called the anode electrode, and the electrode to which an oxidant gas is supplied is called the cathode electrode. A diagrammatic cross sectional view showing the structure of a unit cell in such a conventional polymer electrolyte fuel cell is shown in FIG. 1.
In FIG. 1, disposed in intimate contact with both sides of a hydrogen ion conductive polymer electrolyte membrane 11 which selectively transports hydrogen ions are catalyst layers 12 whose main component is carbon powder carrying a platinum-type metal catalyst. Further disposed in intimate contact with the outer surfaces of the catalyst layers 12 are a pair of gas diffusion layers 13 composed of a porous material having pores. The gas diffusion layer 13 and the catalyst layer 12 constitute a gas diffusion electrode 14.
Outsides of the gas diffusion electrodes 14 are disposed separator plates 17 which mechanically fix the electrolyte membrane-electrode assembly 15 (hereinafter referred to also as “MEA”) composed of the gas diffusion electrode 14 and the hydrogen ion conductive polymer electrolyte membrane 11, and electrically connect adjacent MEAs in series. The separator plate 17 is provided with a gas flow path 16 at one side for supplying the gas diffusion electrode 13 with a fuel gas as a reaction gas or an oxidant gas, and carrying away the water content produced in the reaction and a surplus gas. This gas flow path 16 can be formed by causing an additional member adhered on the separator plate 17; however, in general, a groove is formed on the surface of the separator plate by a cutting process to form the gas flow path.
In the cathode electrode of the fuel cell in operation, an oxidant gas such as air or oxygen, which is a reaction active material, is diffused into the catalyst layer through the gas flow path via the gas diffusion layer 13. The surplus water produced in the reaction and penetrated from the catalyst layer to the gas diffusion layer is discharged outside the cell through the pores of the gas diffusion layers together with the surplus gas.
The above-described polymer electrolyte fuel cell has the property that the hydrogen ion conductive polymer electrolyte membrane 11 has a higher degree of ion conductivity with increasing moisture content, which makes it necessary to keep the hydrogen ion conductive polymer electrolyte membrane 11 in the humidified condition. For this reason, it is general to humidify the reaction gas in advance to have the predetermined moisture level, thereby securing the moisture retention of the hydrogen ion conductive polymer electrolyte membrane 11 at the same time as the supply of the reaction gas.
The water produced as the result of the electrode reaction is flown from the inlet side to the outlet side of the gas flow path together with the reaction gas flowing through the gas flow path of the separator plate, and is discharged outside the fuel cell in the end. Therefore, the moisture content of the reaction gas in the fuel cell differs depending on the position in the flowing direction of the reaction gas in the gas flow path, which means that the reaction gas on the outlet side contains more moisture content and is more humid than the reaction gas on the inlet side of the gas flow path by the amount of water produced by the reaction.
Therefore, in the vicinity of the outlet side of the gas flow path, the function of discharging water from the gas diffusion layer deteriorates, and in the extreme case, there is a problem that the surplus water blocks the pores of the gas diffusion layer, thereby preventing the diffisability of the reaction gas so as to remarkably decrease the cell voltage (flooding phenomenon). In contrast, there is another problem that when a reaction gas whose humidity has been decreased is supplied from the inlet side in order to prevent the occurrence of flooding in the outlet side, the moisture content of the hydrogen ion conductive polymer electrolyte membrane decreases in the vicinity of the inlet side, which increases the conductive resistance of the hydrogen ions, thereby decreasing the cell voltage. These tendencies become remarkable when the electrode has a larger area and when the separator plate has a longer gas flow path.
In view of the above-described prior art problems, the present invention has an object of providing a gas diffusion electrode capable of keeping the water content homogeneous over the entire MEA surface, and also providing a fuel cell capable of stable operation for a long time of period.
In general, the catalyst layer in a gas diffusion electrode is formed as follows. A catalyst ink is prepared by mixing a dispersion medium such as water or isopropyl alcohol into a solution or a dispersion solution, which contains carbon fine powder carrying a noble metal and a polymer electrolyte having hydrogen ion conductivity. This catalyst ink is applied on a porous material such as carbon paper or carbon cloth, which is to be a base material of the electrode, by using a screen printing method or a spray method, followed by drying or baking to form the catalyst layer. Two gas diffusion electrodes each having the gas diffusion layer and catalyst layer thus prepared are connected to each other via an electrolyte membrane by means of a hot press so as to obtain an electrolyte membrane-electrode assembly (MEA). Besides this method, there is considered a method where a catalyst ink is applied on a polymer film or the like by a gravure printing method or a coater method and dried to form a catalyst layer, and then the catalyst layer is transferred onto an electrolyte membrane.
As described above, it is general to make the catalyst layer used in a fuel cell dense as much as possible so as not to have cavities such as cracks on the plane of the catalyst layer, thereby increasing the utilization of the catalyst. For this reason, a surfactant or the like is added to the catalyst layer ink in order to prevent the agglomeration of the carbon particles and to improve the dispersibility of the carbon particles carrying the catalyst, and the particle size of the carbon particles is minimized as less as possible. For example, there is employed a method where the particle size of the carbon particles is minimized by using a triturating device having a high triturating force such as a planetary ball mill. The occurrence of cracks is prevented by carrying out the drying process of the applied catalyst layer ink moderately at a possibly lowest temperature over a long time.
On the other hand, the smooth proceeding of the electrode reaction in the catalyst layer requires the efficient supply of the reaction gas into the catalyst layer. To achieve this, a method is used where a catalyst layer ink containing a pore-forming agent is applied and baked so as to form a catalyst layer having micro-level pores.
It is effective from the viewpoint of the utilization of the catalyst theelf to make the catalyst layer dense as much as possible by adding a surfactant to the catalyst ink, thereby improving the diffusibility and to decrease the particle size of the carbon particles carrying the catalyst particles. However, such a dense catalyst layer is poor in gas diffusibility, particularly in the direction of thickness. When a polymer electrolyte fuel cell is operated at high current density, a large amount of water, which is a reaction product, generates and resides. This causes the problem of preventing the reaction gas from being diffused into the catalyst layer, making it impossible to obtain sufficient cell performance.
Furthermore, in the case of adding a surfactant, the baking process must be done after the application of the catalyst layer ink, which increases the number of process and makes the producing procedure complicated. Although it is necessary to reduce the particle size of the carbon particles by a planetary ball mill or the like, too small carbon particles make the catalyst layer too dense as described above, thereby deteriorating the cell performance.
It is also possible to improve the gas diffusibility by adding a pore-forming agent to the catalyst layer ink; however, this requires the catalyst layer to be baked after being applied and dried, which is not preferable because the number of the process is increased, and the manufacturing procedure complicated. The case of adding a pore-forming agent has another problem that the catalyst layer becomes thicker.
Drying the applied catalyst layer ink at a possibly lowest possible temperature over a long period of time makes a drying apparatus larger, is not preferable in terms of cost reduction and the simplification of the manufacturing procedure, which is not.
In view of the above situations, a method has been demanded for manufacturing a catalyst layer capable of keeping the gas diffusibility while possibly minimizing a decrease in the catalyst utilization of the catalyst in the catalyst layer.